1. Field of the Invention
The present invention generally relates to a method for changing a wireless channel established between a base station and a mobile station to another wireless channel. The present invention also relates to a mobile communication system carrying out the method.
2. Description of the Related Art
The mobile communication system, such as a cellular phone system, which has recently become commonly spread, provides a service by partitioning an overall service area into significantly small wireless zones referred to as “cells”. The mobile communication system includes a plurality of base stations, each of which covers each partitioned wireless zone and a mobile station, which communicates with the base station. The base station and the mobile station communicate with each other by means of a wireless channel established between the base and mobile stations.
In general, the wireless channel between the base and mobile stations are differently established according to a communication style between them. In a PDC (Personal Digital Cellular) system, for example, which is widely spread in Japan, for a narrowband audio communication technique known as “a half rate”, one of time slots is used as the wireless channel where the one time slot is derived from a wireless carrier by time dividing the one wireless carrier into 6 components. The wireless carrier is one of radio frequency resources. For a high quality audio communication known as “a full rate”, two of the time slots derived by time dividing the one wireless carrier are used as the wireless channel.
In a packet communication system, since one wireless carrier is shared among a plurality of user's mobile stations, an amount of data transmitted from each mobile station is reduced in order to coexist with other mobile stations when the mobile stations transmitting the data compete with each other. On the contrary, the wireless channel is configured so that one mobile station can occupy the wireless carrier when no other mobile stations transmit the data.
In a W-CDMA (Wideband Code Division Multiple Access) system, which has been researched and developed as one of possible third generation mobile communication systems, the wireless channel comprises the wireless carriers and spreading codes. Various types of the wireless channels are provided in accordance with information type, such as audio or data, to be transmitted. For data transmission, the wireless channel to be used changes as a transmission rate varies. Different wireless channels can coexist altogether by employing a different spreading coding each other.
A specification, TS 25.213 “Spreading and Modulation (FDD)”, formulated by 3GPP (Third Generation Partnership Project), which is a standardization organization, specifies an assignment of the spreading codes in the third generation mobile communication system. The W-CDMA system also uses different wireless channels or spreading codes in accordance with the communication style between the base and mobile stations.
Thus, in the mobile communication system, since the available radio frequency resources are limited, the use of the appropriate wireless channel in conformity with the communication style is significantly important in order to achieve a higher quality of communication.
For the packet communication system, the amount of outgoing data continuously varies. As a result, the data is continuously delivered when transmitting a large amount of data and the least possible amount of data required for holding a connection is intermittently delivered when there are no data to be transferred. In an ISDN (Integrated Services Digital Network) system for a fixed communication system, a BOD (Bandwidth On Demand) technique is employed to achieve efficient link availability.
The BOD basically uses one B channel having a transmission capacity of 64 kbps in case of an ordinary condition. The BOD technique controls the use of the B channels depending on the amount of data to be transferred. For example, it reserves a bundle of a plurality of B channels in order to achieve higher transmission capacity when the amount of data to be transferred increases. Then, as the amount of data to be transferred decreases, the BOD sequentially relieves the B channel from the reserved bundle of the B channels and eventually the number of the B channels used by the BOD returns to one.
In general, such control of the use of the B channels is carried out when two terminals communicate or the terminal and a switching facility communicate with each other. If the bundle of the B channels were regularly used, the communication through the bundle of the B channels would cost much higher because connection fees are charged on the basis of one channel. However, in the BOD technique, the communication cost will be reduced because the number of the B channels being used varies as a function of the data to be transferred. Furthermore, the efficient use of the connections is achieved in the BOD environment. It can be appreciated that the BOD technique is a suitable control scheme for the packet communication where a data transfer frequency continuously changes.
However, an application of the above BOD technique to the base and mobile stations in the mobile communication system comes across several problems. Firstly, since a variety of information for keeping on the wireless channel must be carried on the wireless channel, the transmission capacity of the wireless channel reduces. For example, the information includes pilot signals used for correctly receiving signals subject to a phase rotation and/or an amplitude variation due to a fading along a transmission path, commands for transmission power control, and so on. Such information is essential in the mobile communication system, which is involved in significantly variable transmission conditions, whereas the information adversely effects on the transmission efficiency.
Under the mobile communication environment, if the mobile station uses the bundle of the plurality of the wireless channels, each wireless channel of the bundle transmits the same information and resulting in reduction of the transmission efficiency. This causes not only a problem that the mobile station cannot achieve the desired transmission efficiency, but also a further problem that the total transmission efficiency over the system is reduced because the number of the wireless channels available to the other mobile stations decreases.
Secondarily, even if the wireless channel or the spreading code is available to the mobile station, the use of such wireless channel can increase the radio interference with the other mobile stations and thus degrading the total transmission capacity over the system. This is because, for example, in the PDC system, the mobile station trying to use that wireless channel may be interfered with the same wireless channel that has been repeatedly used by the other mobile station/stations.
For W-CDMA system, Japanese Laid-Open Patent Application No.8-191481 discloses a method for determining an acceptance of a new call based on a prediction of interference quantity at the base station. The document describes the importance of the interference quantity on an uplink of the system in detail. International Patent Publication WO98/30057, titled “CALL ACCEPTANCE CONTROL METHO FOR CDMA MOBILE RADIO COMMUNICATION SYSTEM AND MOBILE STATION DEVICE”, describes a method for determining whether the call is accepted or not in the mobile station by reporting information concerning the uplink interference quantity and a downlink transmission power from the base station to the mobile station. Furthermore the latter document describes that the total transmission power is significantly critical with respect to quality on the downlink in detail. In the W-CDMA system, the uplink interference and/or downlink transmission power will often limits the transmission capability of the system.
Therefore, for the mobile communication system, a technique is demanded to improve transmission efficiency without applying the above BOD-like technique.